The DMD gene has 7 tissue-specific promoters
[Muntoni et al., 2003]
summarized in table 1.
The full-length dystrophin transcription is controlled by 3 promoters
localized upstream to the first exon. The muscle (m), brain (c) and Purkinje (p) promoters are named according to their principal
site of expression. The brain promoter is expressed primarily in cortical neurons and in the hippocampus.
The purkinje promoter induces transcription in Purkinje cells but also in skeletal muscle. The muscle promoter induces transcription
predominantly in skeletal and heart muscle but also in some glial cells in the brain.

Furthermore, there are 4 internal promoters, dp260, dp140, dp116 and dp71 (named according to the molecular weight of
the produced protein)
[Sadoulet & Kunkel, 1996].
Dp260 is a retinal specific transcript localized in the outer plexiform layer of the retina
but it is also found in heart and brain. Dp140 is principally found in brain, retina and kidney. Dp116 is a
Schwann-cell promoter. Dp71 transcripts have been found in most non-skeletal muscle tissues such as brain, kidney,
liver, lung and cardiac muscle.

Alternative splicing occurring at the 3'-end of the gene produces an even greater number of transcripts. This differential
splicing may regulate the binding of dystrophin to dystrophin-associated proteins at the membrane
[Blake et al., 2002].

Dp427m is the main variant produced in muscle and is involved in Duchenne and Becker Muscular Dystrophies.
eDystrophin is a database dedicated to this major variant.

Schematic representation of dystrophin structural domains.
First line represents a simplified description of domains as found in
publications. Boxes represent a more detailed view of dystrophin described below.

The full length dystrophin is a skeletal protein comprising several structural domains.
Four major domains are generally described: actin binding domain, rod domain, cysteine-rich domain
and the COOH terminus region [Koenig et al., 1989,
Blake et al., 2002,
Le Rumeur et al., 2010].
In eDystrophin , we use a more detailed description of the protein domains and distinguish structural domains from
binding domains. A structural domain has a tertiary/quaternary structure but has not necessarily a defined function.
A binding domain has a well defined function
and can encompass one or several structural domains. The functional domains are described in the Binding domains page.
Structural and binding domains may have different ends according to different works but most of the time
this ends change only by few amino acids. Usually, we will use the definition according to the most recent publication.

The Actin Binding Domain (ABD)[Norwood et al., 2000]
is composed of two calponin homology domains (CH1 and CH2). These CH domains are two well defined structural domains whose
tri-dimensional structure has been resolved by X-ray crystallography.

The rod domain is the central domain of the protein which represents about 75% of the dystrophin.
This domain is composed of 24 repeats (R1 to R24) homologue to spectrin repeats and 4 hinges (H1 to H4)
[Koenig et al., 1989,,
Winder et al., 1995]
These hinges share the rod domain into three
sub-domains. Repeats can be aligned according to a heptad motif and they may fold into three alpha-helices which constitute
left-handed antiparallel triple-helical coiled coils.
Today, no structure of dystrophin repeats is available, probably because of their high conformational flexibility.

Alignment of spectrin-type triple-helical repeats in dystrophin, colour coded to highlight conserved features from
Winder et al., 1995.
Gaps in unconserved loops have been minimised and deleted sequences are recorded within <>.
*Indicates aberrant repeats with truncated helices. Comment lines below the sequence annotate the 3 a-helices;
heptad, the helical heptad periodicity;
helices, helix name for identification purposes, it should be noted that helix C is continuous with helix A
in the following repeat; Spc no., residue number as for the solved structure of a Drosophila repeat [20].
All G (orange) and P (yellow) residues are coloured. Other colouring is by conserved property in >55% of any column:
uncoloured residues lack a sufficiently conserved property. Blue, hydrophobic; light blue, partially hydrophobic;
red and pink, positive; purple, negative; green, hydrophilic.

The cysteine-rich domain contains a high number of cysteine residues and overlaps with the end
of WW domain, the two EF hands and the zinc finger domain (ZZ).
The WW domain is defined by the presence of two tryptophan residues which are spaced by 21 amino acids.
This domain is followed by an alpha-helix and two EF-hand-like domains. An EF hand is a helix-loop-helix motif
found in many calcium binding proteins. However, no calcium binding has yet been reported in dystrophin EF hands
and it seems that dystrophin may have lost this property. All these domains have been crystallized and their
tri-dimensional structure resolved by X-ray crystallography
[Huang et al., 2000].
These domains are followed by a zinc finger domain.
These four domains are involved in the binding of β-dystroglycan to dystrophin (see here).

The COOH Terminus consists of two polypeptides which form a coiled-coil with a conserved repeated
heptad pattern of the residues similar to the rod domain repeats.

Dystrophin is a skeletal-muscle protein that confers resistance to the sarcolemma against the stress
of contraction-relaxation cycles by connecting various cytoskeletal elements such as actin and
micro-tubules to the intrinsic protein, β dystroglycan.

ABD1 & ABD2 - Actin binding property: The two CH domains in tandem constitute the Actin-Binding Site 1 of dystrophin
[Banuelos et. al, 1998,
Norwood et. al, 2000]. CH1 has a strong
affinity to actin whereas CH2 enhances the binding affinity but alone does not bind to F-actin. Interaction with
F-actin is localized in the hydophobic groove identified on the surface of CH1 which extends into the CH2.
The second Actin-Binding Domain encompasses the repeats R11 to R17 and interacts with F-actin through an electrostatic
interaction [Amann et al, 1999].
It does not display any binding activity on G-actin.

LBD1 & LBD2 - Lipid binding property: Part of the rod domain of dystrophin is a partner of anionic membrane phospholipids
[Legardinier et al., 2008 and
Vié et al., 2010].
The functional domain is divided into two parts: one lipid binding domain (LBD1) comprises the repeats R1 to R3
whereas a second lipid binding domain (LBD2) comprises repeats R4 to R19. This interaction places dystrophin very near
the sarcolemma with a large part of the central rod domain lying along the phospholipid membrane bilayer.

PAR-1b binding domain: PAR-1b is a cell polarity kinase. Recent reports have revealed interactions
between repeats 8 and 9 of dystrophin and utrophin with Par-1b kinase. In both cases the kinase appears to
phosphorylate the repeats [Yamashita et al., 2009].

Synemin binding: Dystrophin can bind intermediate filaments through a direct binding with synemin,
one of the proteins of the intermediate filaments
[Bhosle et al., 2006]. A first synemin binding domain
is localized on the rod domain, more specifically on repeats R11 to R14, which overlaps with the ABD2.
A second binding domain encompasses the WW to ZZ domain.

Plectin binding property Plectin, a large cytolinker protein that can connect actin filaments,
intermediate filaments and microtubules, has also been shown to make interactions with dystrophin
with the cys-rich domain, therefore providing an indirect link with the intermediate filament protein desmin
[Rezniczek et al., 2007].

βDG - β-Dystroglycan binding Dystrophin interacts with β-Dystroglycan through the WW domain,
the two EF hands, and the ZZ domain. A structure of β-DG together with the dystrophin WW, EFH1 & EFH2
has been resolved by X-ray crystallization by
Huang et al., 2000. Thereafter,
Hnia et al., 2007 and
Ishikawa et al., 2004 showed that β-DG has only
a reduced binding activity with the WW and EF hands of dystrophin and that the addition of the ZZ domains restored
a high binding activity of β-DG to dystrophin.

Syntrophin and Dystrobrevin binding These proteins are two dystrophin-related proteins whose C-termini
are similar to the C terminus homologous with the
C terminus of dystrophin. Both are able to combine with the first helix of the dystrophin coiled-coil C terminus
motif [Sadoulet et al., 1997,
Newey et al., 2000].

Myospryn binding Myospryn is a muscle-specific protein kinase A (PKA) anchoring protein or AKAP. Myospryn
interacts with dystrophin through the WW domain, the two EF hands, and the ZZ domain
[Reynolds et al., 2008].

Mutations in the DMD gene can lead to two major diseases: Duchenne Muscular Dystrophy (DMD) and
Becker Muscular Dystrophy (BMD). In most of cases, DMD results from a frame shift mutation
which leads to mRNA decay and a total absence of dystrophin in muscle cells. On the other hand, BMD results
from an in-frame mutation which leads to an internally truncated dystrophin that is threfore only partially functional.
This rule was established by [Monaco et al., 1988]
but there are some exceptions.

Duchenne Muscular Dystrophy (DMD) is an X-linked disease with an incidence of around
1:3,500 male births. Patients are usually confined to a wheelchair before the age of 12 and have a life span
reduced to about 20 years. No dystrophin is observed in the muscle cells of these patients.

Becker Muscular Dystrophy (BMD) is a less severe X-linked disease leading to highly variable phenotypes
from patients with only weakness to patients confined to a wheelchair. An internally truncated dystrophin is observed
in muscle cells with very usually a decrease in the protein level compared with normal humans. As the dystrophy is less
severe as a whole compared with DMD, it appears that these mutated dystrophins remain partially functional and
able to somewhat compensate for the function of full length dystrophin.

Intermediate Muscular Dystrophy (IMD) can be observed in some patients. These patients have an intermediate phenotype
and are confined to a wheelchair between the ages of 13 to 16.

X-linked cardiomyopathy is an isolated cardiomopathy. These patients have no skeletal-muscle disorders.

Therapies

Injection of DMD gene is the initial approach for gene therapy strategy to envisage.
A truncated gene is encapsuled in
a recombinant adeno-associated virus and injected into patients
[Gregorevic et al., 2006].
Then, a functional truncated dystrophin is produced by the patient's cells. The size of the delivered gene is
limited by the vector size and therefore reduced DMD genes (mini- or micro-gene) have been constructed
to be encapsulated in recombinant adeno-associated viruses. These mini- or micro-genes may be able to
produce partially functional internally truncated dystrophins such as those observed in BMD.

Exemple of gene therapy with a truncated DMD gene inserted.

Exon skipping therapy is one of the most promising experimental therapies for Duchenne Muscular Dystrophy
[Beroud et al., 2007],
[Arnett et al., 2009].
This skipping restores a true reading frame leading to the synthesis of a dystrophin deleted for
part of the central coiled-coil region as observed in BMD patients. This strategy should transform a DMD patient
into a BMD patient
[Goyenvalle et al., 2004,
Aartsma-Rus et al., 2002].
Antisense oligonucleotides (AO) are used to alter gene expression by skipping one or more exons in
the final transcript with different associated strategies
[Wilton & Fletcher, 2008,
Goyenvalle et al., 2010].
However, this strategy has some limitations. AOs can be used to skip all single exons except
the first and last exons.
The exon skipping strategy is mutation-specific but
some exon skipping strategies could be applicable to large groups of patients
[Aartsma-Rus et al., 2009].
The mode of administration is not yet well defined to restore dystrophin expression particularly in
cardiac muscle.

Cell therapy is an alternative therapy with the aim of restoring a pool of normal muscle cells
in the skeletal or heart muscles. First, a transplantation of adult myoblasts is
performed but these cells have only a limited survival time. Now, dissemination of skeletal muscle precursors
being studied.

Exemple of exon skipping by skipping exon 51 on a deletion from exons 45 to 50